The present invention relates to a liquid crystal display panel driving apparatus, and to a liquid crystal display apparatus to be driven by the same.
In recent years, liquid crystal display (hereinafter referred to as “LCD”) panel driving apparatuses for converting input digital image data into analog image data according to a plurality of reference voltages and displaying the image on an LCD panel, as well as LCD apparatuses driven by such driving apparatus, have become popularly available in the market.
An example of a conventional LCD panel driving apparatus and an LCD panel driven by the same will be described referring to FIG. 5 to FIG. 8. FIG. 5 is a block diagram showing a configuration of a conventional digital-input LCD apparatus. In FIG. 5, the numeral 501 indicates a gate driver for generating a vertical timing for writing data and the like, numeral 502 indicates an LCD panel constituted of a thin film transistor and so on, and numeral 503 indicates a source driver for writing horizontal image data into the LCD panel.
The digital-input LCD apparatus constituted as above operates as follows. The source driver 503 is provided with digital image data (6 bits each of RGB in this example) and a plurality of reference voltage values ((n+1) points in this example, where n is an arbitrary positive integer) ranging from a ground potential to a source potential E0. A D/A converter in the source driver converts a digital value representing image data into a corresponding analog voltage value according to a bending line approximating a reference voltage values (Vref(0) to Vref(n) in this example), and outputs the analog voltage value. The gate driver 501 generates a timing signal for writing the data in a thin film transistor and the like of the LCD panel 502, so that the analog image data output by the source driver 503 is written in the LCD panel according to the timing signal.
The LCD panel is usually driven by AC (alternating current), in such a manner that a voltage applied to the LCD panel is inverted at each horizontal scanning period and each vertical scanning period. The Japanese Unexamined Laid Open Gazette Hei 07-219482 proposes direct inversion of digital data itself to be input to an LCD panel, as the simplest method of inverting a voltage to be applied to an LCD panel.
FIG. 6 is a line graph showing voltage values output by the source driver 503 corresponding to both directly inverted and non-inverted digital image data input thereto. In FIG. 6, the horizontal axis indicates the digital data, while the vertical axis indicates the voltage value. According to FIG. 6, in case where the digital image data itself is inverted, a conversion characteristic from inverted digital data to output voltage value is shown by a bending line made by making the bending line representing a conversion characteristic from non-inverted digital data to output voltage value folded back at a median value of the digital data (a bending line laterally folded back at a center of the horizontal axis).
Focusing on the output voltage corresponding to the respective digital values, the conversion characteristic from non-inverted digital data to output voltage value and that from inverted digital data to output voltage value do not constitute vertically symmetrical bending lines with respect to the median point E0/2 of the voltage (The conversion characteristic from inverted digital data to output voltage value deviates from an ideal curve shown in FIG. 6.). Therefore, a completely DC-free AC driving cannot be performed for the LCD panel.
In case where a DC component is contained in a voltage to be applied to an LCD panel, performance thereof is prone to cause the LCD panel degraded, thereby shortening its life span. Accordingly, a set of a plurality of reference voltages Vref(k) (k=0 to n) approximating the bending lines in the graph has been designed such that a DC component is excluded from a voltage to be applied to an LCD panel, to a substantially equal extent in average over various types of screen images.
On the other hand, a conversion characteristic from digital data to output voltage value, which constitutes a so-called gamma curve, is an important factor in creating a visual impression given to a user through an LCD panel. However, the necessity of excluding a DC component from a voltage to be applied to an LCD panel when constituting a set of reference voltages Vref(k) (k=0 to n) has been restricting the designing freedom of the conversion characteristic from digital data to output voltage value that provides a user with a most preferable visual effect.
Some of the latest digital-input LCD panels are designed such that a plurality of reference voltage values are input to an internal D/A converter, which converts the input data to analog data according to a conversion characteristic approximating a bending line formed based on the reference voltage values, and the reference voltage value is inverted so that the analog data converted by the D/A converter is inverted.
FIG. 7 is a bending line graph showing a conversion characteristic achieved by such LCD panel with respect to inverted and non-inverted reference voltages Vref(k) (k=0 to n). In FIG. 7, the horizontal axis indicates the digital data, while the vertical axis indicates the voltage value. In this case, since the voltage itself of the Vref(n) (n=0 to 10) is inverted, the line forms a vertically symmetrical waveshape, thereby achieving a substantially ideal waveshape of inverted and non-inverted voltages.
FIG. 8 is a circuit diagram of a reference voltage generating unit popularly used for inverting the reference voltage. In FIG. 8, the numerals R100 to R10n and R200 to R20n represent individual resistors in separate resistors groups, one end of which is connected to a power source and the other end to a ground in opposite polarities between each other. Resistance values are set as R100=R200, R101=R201, and R10n=R20n respectively. Selectors 0 to n are selector switches for alternately selecting a voltage value divided by resistors R100 to R10n and a voltage value divided by resistors R200 to R20n for each horizontal scanning period. A conventional reference voltage generating unit is constituted as above. Referring to output terminals Vref(0) to Vref(n), a voltage corresponding to a bent point of a non-inverted voltage line and a voltage corresponding to a bent point of an inverted voltage line shown in FIG. 7 are alternately output from these terminals at each horizontal scanning period.
Though the reference voltage generating unit according to FIG. 8 can symmetrically output the inverted and non-inverted voltage values, such unit requires two resistor systems for obtaining predetermined voltages, and the same number of selector switches as that of the voltages to be provided, which inevitably incurs an increase of the circuit scale.
It is an object of the present invention to provide an LCD panel driving apparatus that can inexpensively drive a digital-input LCD panel by employing a circuit capable of inverting a reference voltage without incurring an increase of the circuit scale, and an LCD apparatus to be driven by such driving apparatus.